1. Field of the Invention
This invention is directed to systems and methods for determining information about objects, such as their shape, size, orientation and the like, and for systems and methods for controlling one or more processes or devices based on the determined object information.
2. Related Art
Suspension crystallization processes often result in crystals having a high aspect ratio. Such high-aspect-ratio crystals are commonly described as needle-like, rod-like or acicular. Such high-aspect-ratio crystals are particularly common-place in the types of complex, high-value-added chemical compounds typically produced by the specialty chemical and pharmaceutical industries. When crystallizing such high-aspect-ratio, crystal-forming chemical compounds, controlling the particle size distribution (PSD) is typically highly important. For example, when the particle size distribution of such high-aspect-ratio crystals is within a desired range, the efficiency of the downstream manufacturing process may be optimized or maximized. Likewise, when the particle size distribution of such high-aspect-ratio crystals is within a desired range, the overall quality of the end product being made, whether such high-aspect-ratio crystals are an intermediate product or the final end product, may be optimized or maximized.
Conventional techniques for determining the particle size distribution of a group of crystals include laser diffraction and laser backscattering, which are commonly-used on-line techniques. It should be appreciated that the drawbacks associated with these techniques discussed below are characteristic of the drawbacks associated with other techniques. T. Allen, “Particle Size Measurement, Vol. 1, 5th Edition”, Chapman and Hall, London, 1997, discusses the conventional techniques in detail.
Laser diffraction operates by passing laser light through a quantity of the suspended crystal particles. The diffracted laser beams are diffracted onto a CCD array or the like, where the diffraction patterns are captured. Based on the captured diffraction patterns, the crystal size and particle size distribution can be determined. However, the analysis algorithms developed for analyzing the diffracted patterns have all been developed based on the assumption that the particles are spherical. Spherical particles make the analysis easy, because the diffraction patterns are independent of the orientation of the crystal particles and thus are solely dependent on the size of crystal particles. Such orientation independence is obviously an appropriate assumption only for spherical particles, or near spherical particles, such as tetrahedrons, cubes and other near spherical particles.
Because the measured size of high-aspect-ratio crystals is highly dependent on the orientation of the particles, such laser diffraction methods are inappropriate for high-aspect-ratio crystals. Additionally, because the diffraction patterns are formed by passing light through a sample, such diffraction patterns are typically inappropriate for in situ measurements or measurements of crystal solutions having high solids concentrations, where an insufficient amount of light would actually pass through the sample and be recorded. Thus, laser diffraction over-estimates the broadness of the spherical diameter distribution for high-aspect-ratio crystals due to such orientation effects and the spherical models used to interpret the diffraction data.
In contrast to laser diffraction, which relies on light passing through the sample, laser backscattering relies on the particles reflecting a sufficient amount of light back towards the light source. Laser backscattering provides a cord length distribution that can be related theoretically to the particle size distribution. In laser backscattering, the laser beam is rotated over the particle slurry such that each particle backscatters light as the light passes over that particle. Based on a time-to-cross measurement and the known speed of movement of the laser beam, the cord length of the laser beam's path over the crystal can be determined. The cord length distribution can only be related back to the actual size distribution of the crystals by assuming some geometry for the particles, such as aspect ratio, orientation and/or the like. Because the aspect ratio, i.e., a length to thickness, of the crystals is one of the variables that appropriate control of the crystallization process affects, assumptions about the crystals' geometry render the analysis less than complete.
As a result of the shortcomings of laser diffraction and laser backscattering, various imaging-based systems have been developed to size high-aspect-ratio, i.e., elongated, crystals. Such imaging systems and techniques offer the potential to extract both size and shape information. Thus, such imaging based systems and techniques are a promising and attractive approach for obtaining particle size distributions for non-spherical particles. Conventional, imaging-based, on-line particle size and shape analyzers are available from Malvern and Beckman-Coulter, such as the Malvern Sysmex FPIA3000 and the Beckman-Coulter RapidVUE. Powder Sampling and Particle Size Determination, by T. Allen, Elsevier, 2003, surveys other imaging-based instruments.
Typically, these instruments require withdrawing a sample of the crystal slurry from the crystallization reaction vessel. Drawing such samples is inconvenient, possibly hazardous, and raises concerns about whether the sample is truly representative of the bulk slurry. One notable system that provides for in situ sampling is the Particle Vision and Measurement (PVM) system from Lasentec, Inc. The Lasentec Particle Vision and Measurement in situ probe is combined with automatic image analysis software that is useful for some types of crystals. However, this system does not give suitable results for high-aspect-ratio crystal particles.